This application is a continuation-in-part of patent application Ser. No. 226,648 filed on Aug. 1, 1988.
The invention disclosed herein relates to molded case circuit breakers. The following seven patent applications all relate to molded case circuit breakers and were filed on Aug. 1, 1988: Ser. No. 226,500, entitled Rubber Stops in Outside Poles, William E. Beatty, Jr., Lawrence J. Kapples, Lance Gula and Joseph F. Changle; Ser. No. 226,503, entitled Cross-Bar Assembly, by Jere L. McKee, Lance Gula, and Glenn R. Thomas, Ser. No. 226,649, entitled Laminated Copper Assembly, by Charles R. Paton; Ser. No. 226,650, entitled Cam Roll Pin Assembly, by Lance Gula and Jere L. McKee; Ser. No. 226,655, entitled Combination Barrier and Auxiliary CT Board by Gregg Nissly, Allen B. Shimp and Lance Gula; and Ser. No. 226,654, entitled Modular Option Deck Assembly by Andrew J. Male.
The following four commonly assigned U.S. patent applications were filed on Oct. 12, 1988 and all relate to molded case circuit breakers: Ser. No. 256,811entitled Screw Adjustable Clinch Joint With Bosses, by James N. Altenhof, Ronald W. Crookston, Walter V. Bratkowski, and J. Warren Barkell, Ser. No. 256,879 entitled Tapered Stationary Contact Line Copper, by Ronald W. Crookston, Ser. No. 256,880, entitled Side Plate Tapered Twist-Tab Fastening Device for Fastening Side Plates to The Base, by K. Livesey and Alfred E. Mainer, and Ser. No. 256,878, entitled Two-Piece Cradle Latch For Circuit Breaker, by Alfred E. Maier and William G. Eberts.
The following commonly assigned U. S. patent applications also relate to molded case circuit breakers: Ser. No. 260,848, filed on Oct. 21, 1988, entitled Unriveted Upper Line Securement, by Joseph Changle and Lance Gula, Ser. No. 07/331,769, filed on Apr. 3, l989, entitled Arc Runner Containment Support Assembly, by Charles Paton, Kurt Grunert and Glen Sisson, and Ser. No. 07/331,920, filed on Mar. 31, 1989, entitled Extender Spring for Increased Magnetic Trip Settings, by Curt Grunert.
The following two commonly owned patent applications were filed on Apr. 25, 1989: Ser. No. 07/343,047, entitled Two-Piece Cradle Latch, Key Blocks and Slot Motor for Circuit Breaker, by Alfred E. Maier, William G. Eberts and Richard E. White, and Ser. No. 07/342,820, entitled Two-Piece Cradle Latch, Handle Barrier Locking Insert and Cover Interlock for Circuit Breaker by A. D. Carothers, D. A. Parks, R. E. White and W. G. Eberts.
Commonly owned patent application Ser. No. 07/374,370 was filed on June 30, 1989, entitled Reverse Switching Means for Motor Operator, by Kurt Grunert and Charles Paton.
Lastly, commonly owned Ser. No. 389,849, was filed on Aug. 4, 1989, entitled Trip Interlock Design, by Kurt A. Grunert, Ronald A. Cheski, Michael J. Whipple, Melvin A. Carrodus, James G. Mahoney and Robert J. Tedesco.
1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to a quick change assembly for the main current transformers to allow the main current transformers to be quickly and easily replaced in the field and to means for reducing the time required for the separable main contacts to blow open at a predetermined level of overcurrent.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U. S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit. An overload condition is about 200-300 % of the normal current rating of the circuit breaker. A high level short circuit condition can be 1,000% or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, the contacts may be open by an operating mechanism or by a magnetic repulsion member. The magnetic repulsion member causes the contacts to separate under relatively high level short circuit conditions. More particularly, the magnetic repulsion member is connected between a pivotally mounted contact arm and a stationary conductor. The magnetic repulsion member is a generally V-shaped member including a bight portion and two depending legs defining a parallel current path. During high level short circuit conditions, magnetic repulsion forces are generated between the depending legs of the magnetic repulsion member as a result of the current flowing in opposite directions in the parallel current path which, in turn, causes the pivotally mounted contact arm to open.
In a multipole circuit breaker, such as a three-pole circuit breaker, three separate contact assemblies having magnetic repulsion members are provided; one for each pole. The contact arm assemblies are operated independently by the magnetic repulsion members. For example, for a high level short circuit on the A phase, only the A phase contacts would be blown open by its respective magnetic repulsion member. The magnetic repulsion members for the B and C phases would be unaffected by the operation of the A phase contact assembly. The circuit breaker operating mechanism is used to trip the other two poles in such a situation. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and feed the fault.
In the other automatic mode of operation, the contact assemblies for all three poles are tripped together by a current sensing circuit and a mechanical operating mechanism. More particularly, current transformers are provided within the circuit breaker housing to sense overcurrent conditions. When an overcurrent condition is sensed, the current transformers provide a signal to either an electronic trip unit or an electromechanical trip unit which actuates the operating mechanism to cause the contacts to be separated.
Oftentimes it is necessary to remove a current transformer after the circuit breaker has been assembled. There are various reasons for replacing a current transformer. One reason is that the originally installed current transformer may be defective. Another reason for replacing a current transformer is that the wrong current transformer may have been installed. Moreover, in order to change from one rating to the other rating of a dual rating circuit breaker, for example, 1600/2000 ampere, it may be necessary to replace the current transformer. Lastly, some circuit breakers may be used as a switch obviating the need for a current transformer.
In conventional circuit breakers, the replacement of a current transformer in the field is a difficult and time consuming task. More particularly, the replacement requires extensive dismantling of the circuit breaker in the field resulting in relatively high labor costs and costly down time.
Another problem with known circuit breakers is the need to reduce the time required for the separable contacts to blow open. In some known circuit breakers a generally V-shaped magnetic repulsion member or shunt defining a pair of depending legs is connected between the movable contact and the load-side conductor. The V-shaped shunt is flexible and acts like a spring.
During relatively high level overcurrent conditions, magnetic repulsion forces are generated between the depending legs of the shunt due to electrical current flowing in opposite directions in the depending legs. These magnetic repulsion forces are a function of the distance between the depending legs of the shunt and the magnitude of the electrical current flowing therethrough. In order to develop sufficient magnetic repulsion forces between the two depending legs of the shunt, it is necessary that the shunt be compressed (e.g., distance between the depending legs decreased) to generate sufficient magnetic repulsion forces between two depending legs of the shunt to blow the pivotally mounted contact arm open. The shunt is compressed by magnetic repulsion forces developed between the depending leg adjacent the load-side conductor of the shunt and the load-side conductor. This compression force forces the depending legs closer together to allow sufficient magnetic repulsion forces to be generated between the two depending legs to blow open the pivotally mounted contact arm. Thus, blow open of the pivotally mounted contact arm is delayed until such time that the sufficient compression forces are developed between the load-side conductor and the depending leg of the shunt adjacent the load-side conductor.